Pre-synaptic Cav2.2 (N-type) voltage-gated calcium channels in the dorsal horn of the spinal cord modulate the release of key pro-nociceptive neurotransmitters such as glutamate, substance P(SP) and calcitonin-gene-related peptide (CGRP), indicating the potential therapeutic use of Cav2.2 calcium channel blockers as analgesics.
Peptidic ω-conotoxins, isolated from the venom of cone snails, have been shown to be selective for Cav2.2 calcium channels and can block SP release in the spinal cord (Smith et al. (2002) Pain, 96: 119-127). Moreover, they have been shown to be antinociceptive in animal models of chronic pain following intrathecal administration (Bowersox et al. (1996) Journal of Pharmacology and Experimental Therapeutics, 279: 1243-1249; Smith et al. (2002) supra), and have been shown to be effective analgesics in clinical use, particularly in the treatment of neuropathic pain (Brose et al. (1997) Clinical Journal of Pain, 13: 256-259).
Winquist et al. has shown that Cav2.2 channels may offer the potential to reduce neuronal signalling, thereby treating disorders such as pain. However, side effect issues may impact the success of such an approach (Winquist et al. (2005) Biochemical Pharmacology, 70: 489-499). A number of journal articles have been published on the effect of natural inhibitors of Cav2.2 channels (see Bowersox et al. (1996) Journal of Pharmacology and Experimental Therapeutics 279(3):1243-1249; Scott et al. (2002) European Journal of Pharmacology 451(3):279-286). In addition, several journal articles have been published on the phenotypic characterisation of transgenic mice lacking the Cav2.2 gene (see Saegusa et al. (2001) EMBO J. 20(10):2349-2356; Kim et al. (2001) Mol. Cell. Neurosci. 18(2):235-245). These articles support the stance that tonic inhibition of Cav2.2 may result in cardiovascular (hypotension) and CNS (sedation) side effects at therapeutic concentrations.
Due to these drawbacks of tonic Cav2.2 inhibitors, it is the object of the invention to provide an alternative class of Cav2.2 antagonist: a state- or use-dependent Cav2.2 blocker, which has the potential to selectively inhibit highly active channels contributing to the pathophysiology of chronic pain whilst sparing the contributions of Cav2.2 to wider physiological levels of activity within the peripheral and central nervous system. Therefore, the object of the invention is to identify novel compounds for use in therapy that preferentially block Cav2.2 calcium channels under conditions of increased neuronal excitability, so-called use-dependent blockers, as is the case in chronic pain syndromes.